The present invention relates generally to angular position methods and devices for motors. More specifically, the present invention relates to a technique and apparatus to calculate the absolute angular position of a synchronous motor elevator machine at standstill by detecting stator iron saturation.
Permanent magnet synchronous machines for elevator systems offer advantages over conventional induction elevator machines in the size required for a given duty. However, elevator systems utilizing synchronous motor elevator machines must be capable of detecting absolute angular rotor position, i.e., rotor magnetic flux d axis position and direction, relative to the stator pole windings to be able to achieve maximum torque.
This is particularly significant when the rotor position is lost due to circumstances such as a power failure. When an elevator experiences a power loss, the elevator brake is engaged to hold the elevator car in position. Once power is reestablished, torque to the elevator machine must be available and controlled when the machine brake is lifted to ensure controlled motion of the elevator car under unbalanced load conditions.
An incremental encoder with one index pulse has been used to establish absolute rotor position on prior art elevator machines. However, this may require up to one full revolution of the elevator machine to locate the index pulse after power loss. In larger elevator systems, one revolution of the elevator machine may result in as much as a one meter drop in the elevator car.
A technique that makes use of the saturation effect of the stator iron to detect the rotor position of a permanent magnet synchronous motor is disclosed in an article titled xe2x80x9cInitial Rotor Angle Detection Of A Non-Salient Pole Permanent Magnet Synchronous Machinexe2x80x9d, published in the Conference Records of the IEEE-Industry Applications Society Annual Meeting, New Orleans, La. Oct. 5-9, 1997 (the article). The article describes a method whereby a broad frequency band voltage pulse, of appropriate magnitude and width, is applied to each phase winding of the stator. A single sample of stator peak current is then measured in the time domain for each winding and used to calculate inductance. Since the inductance will vary with the partial saturation of the stator iron and the flux due to the position of the rotor""s magnets, the algorithm can discern between a north pole and a south pole, and subsequently, the absolute position of the rotor.
However, this technique has inherent sampling issues in a noisy environment, such as an elevator system, that limits the repeatability of the results. This is because the voltage pulse generated is inherently composed of a broad band of frequencies. Therefore, any noise within the frequency band of the voltage pulse, e.g., the switching rate of the transistors in the elevator""s Alternating Current Variable Frequency drive, or any harmonics thereof, effects the accuracy of the readings. Also, with this technique, rotor position is calculated from a single inductance measurement. Therefore one bad sample due to noise can dramatically impact the inductance calculation. The irregular curves of the experimental results shown in the article""s FIG. 6 demonstrate the inherent errors in the inductance measurements, since the expected curves should be smooth sinusiods.
Additionally, in order to obtain an appropriate signal/noise ratio using this technique, significantly large magnitude voltage pulses and peak currents, e.g., at or near the rated current values of the motor, are required. This imposes an undesirable amount of torque on the braking system. In order to compensate for the torque, immediately following the voltage pulse for one phase a voltage pulse in the opposite direction is fired to force the phase currents back to zero. This drives the free wheeling current to zero and helps to minimizes the time torque is applied to the motor.
There is therefore a need for an improved method of detecting absolute angular rotor position relative to the stator windings for a synchronous motor.
In another embodiment of the invention a DC offset current is injected with the AC current into the stator windings. The direction of the d axis is then determined from the minimum of the calculated stator inductances.